Electromechanical dissociation (EMD) is a clinical condition of cardiac arrest with no palpable pulse or blood flow although coordinated ventricular electrical activity exists. This does not exclude the possibility of very weak ventricular contractions, not usually perceptible by standard external or indirect clinical means, which do not produce significant perfusion pressure capable of sustaining life. More recently, the name "Pulseless Electrical Activity" (PEA) has been used for this condition, which may be more descriptive. Since the late 1980's, clinical studies have found a significant number of patients who were judged to be experiencing EMD, but who also had very weak but coordinated mechanical contractions. Some investigators have become dissatisfied with an early definition of EMD as "organized electrical depolarization of the heart without synchronous myocardial fiber shortening and therefore without cardiac output." See, for example, Bocka, J J, et al., Electromechanical dissociation in human beings: An echocardiographical evaluation, Annals of Emergency Medicine, 17:450-452, 1988. "PEA" may therefore better describe a clinical presentation of a cardiac rhythm compatible with adequate perfusion, but producing undetectable or grossly inadequate pulse pressures. For our purposes, however, we will use EMD or electromechanical dissociation to also refer to conditions which are now sometimes called PEA or pulseless electrical activity.
EMD may occur after defibrillation or as the primary modality of cardiac arrest, distinct from fibrillation. Where EMD occurs, the prognosis for recovery is very poor, with survival rates reported on the order of a few percent. When persistent and untreated, EMD is associated with global ischemia, which will result in rapidly progressive and irreversible brain damage within minutes. In the past, the accepted remedy has been to provide continued life-support while waiting for cardiac output to return. Life support measures commonly include CPR and the administration of drugs such as epinephrine in connection with a search for and removal of reversible causes for the condition. Chest compressions and ventilation may extend the period of viability, although it is well known that standard CPR is a poor substitute for cardiac contractions. If cardiac output does not spontaneously return, death is probable. (Cripps, T and J Camm. The management of electromechanical dissociation. Resuscitation. 22(2): 173-180, 1991.) This may particularly be the case for patients with implantable automatic cardiovertor-defibrillators. The device may terminate an episode of ventricular fibrillation, cardiac electrical rhythm may be restored, and the patient may nevertheless die as a result of EMD. (Grubman, EM, et al. Cardiac death and stored electrograms in patients with third generation implantable cardiovertor defibrillators. JACC. 32(4): 1056-1062, 1998.)
The basic implantable cardiovertor/defibrillator system consists of at least one electrode attached to the heart and connected by a flexible lead to a shock or pulse generator. This generator is a combination of a power source and the microelectronics required for the system to perform its intended function. An implantable cardiovertor/defibrillator may also include a pacemaker to treat bradycardia. Many cardiovertor/defibrillators in current use incorporate circuits and antennae to communicate non-invasively with external instruments called programmers. Implantable cardiovertors/defibrillators have the capability of correcting dangerous tachyarrhythmias and fibrillation by applying selected stimulation patterns or high-energy shocks. High-energy shocks are used primarily to correct life-threatening tachyarrhythmias by essentially stopping the heart and allowing an appropriate rhythm to re-establish itself.
There remains a need, however, for apparatus that can not only terminate fibrillation, but can also identify post-defibrillation electromechanical dissociation, and provide a therapy.
It is an object of our invention, therefore, to provide a method for treating post-defibrillation electromechanical dissociation.
It is also an important object of our invention to provide an apparatus that can stimulate the heart in the presence of post-defibrillation EMD in such a manner as to restore life-sustaining cardiac output.
Another important object of our invention is to provide an implantable cardiovertor-defibrillator with apparatus for sensing and treating post-defibrillation EMD.